And now we have to note the important fact which makes this process of development so intensely interesting in relation to the pedigree of the animal kingdom. There is very strong reason to believe that it is a general law of transmission or inheritance, that structural characteristics appear in the growth of a young organism in the order in which those characteristics have been acquired by its ancestors. At first the egg of a dog represents (imperfectly, it is true,) in form and structure the earliest ancestors of the dog; a few days later it has the form and structure of somewhat later ancestors; later still the embryo dog resembles less remote ancestors; until at last it reaches the degree of elaboration proper to its immediate forefathers.
[Illustration: FIG. 1.—An egg: a single corpuscle of protoplasm with nucleus _b_ _c_, and body _a_.]
Accordingly the phases of development or growth of the young are a brief recapitulation of the phases of form through which the ancestors of the young creature have passed. In some animals this recapitulation is more, in others it is less complete. Sometimes the changes are hurried through and disguised, but we find here and there in these histories of growth from the egg most valuable assistance in the attempt to reconstruct the genealogical tree. The history of the development of the common frog is a good illustration of the _kind_ of evidence in question.
[Illustration: FIG. 2.—Tadpoles and young of the Common Frog. 1, Recently hatched (twice natural size); 2 and 2_a_, same enlarged to show the external gills; 3 and 4, later stages with gill slits covered by a membrane leaving only the spiracle (see Fig. 16) as an exit for the respired water; 5, with hind legs appearing; 6, with both fore and hind legs; 7, atrophy of the tail; 8, young frog.]
The frog’s egg first gives rise to a little aquatic creature with external gills and a tail—the tadpole—which gradually loses its gills and its tail and acquires in their place lungs and four legs (Fig. 2), so as now to be fitted for life on dry land. From what we otherwise know of the structure of the frog and the animals to which it is allied, we are justified in concluding that the tadpole is a recapitulative phase of development, and represents to us more or less closely an ancestor of the frog which was provided with gills and tail in the adult state, and possessed neither legs nor lungs.